Mydriatics and Cycloplegics

Mydriatics dilate the pupil and cycloplegics paralyse accommodation. They are used for fundus examination, refraction, to treat uveitis and in some cases to reduce myopia progression. Choice depends on the speed and duration of effect needed.

Mechanism. Antimuscarinic drugs (tropicamide, cyclopentolate, atropine, homatropine) block muscarinic receptors on the iris sphincter and ciliary muscle, causing mydriasis and cycloplegia. Phenylephrine stimulates alpha 1 receptors on the iris dilator muscle, causing mydriasis without cycloplegia.

Pharmacokinetics. Onset and duration vary: tropicamide 20 to 30 minutes and 4 to 6 hours; phenylephrine 30 to 60 minutes and 3 to 5 hours; cyclopentolate 30 to 60 minutes and up to 24 hours; homatropine 1 to 3 days; atropine several days to 2 weeks. Darker irises bind drugs and respond more slowly, so repeat doses may be required.

• Tropicamide 0.5 or 1 percent: short action; routine dilation.
• Phenylephrine 2.5 or 10 percent: adds mydriasis to tropicamide; use 2.5 percent to reduce cardiovascular risk.
• Cyclopentolate 0.5 or 1 percent: cycloplegic refraction in children.
• Atropine 0.5 or 1 percent: long-acting cycloplegia for uveitis and amblyopia treatment; low-dose atropine 0.01 to 0.05 percent for myopia control.
• Homatropine 2 or 5 percent: intermediate action for uveitis.
• Fixed combinations: tropicamide with phenylephrine.

Regimens.

• Fundus examination: tropicamide 1 percent, one drop repeated after 5 minutes if needed; add phenylephrine 2.5 percent if dilation is poor. Examination after 20 to 30 minutes.
• Cycloplegic refraction in children: cyclopentolate 1 percent, one drop repeated after 5 minutes, refraction after 30 to 45 minutes; use 0.5 percent in infants. Atropine 1 percent twice daily for 3 days for strongly hypermetropic or esotropic children.
• Anterior uveitis: cyclopentolate 1 percent or atropine 1 percent two to three times daily (homatropine 2 percent as alternative) to relieve pain and prevent posterior synechiae, together with topical steroid.
• Myopia control: atropine 0.01 to 0.05 percent nightly, where available.
• Break synechiae: atropine with phenylephrine 10 percent.

Selection. Check anterior chamber depth with a torch or slit lamp before dilating; avoid dilation in very narrow angles unless prophylactic iridotomy has been done. Do not dilate before documenting pupil reactions in head injury, stroke or neurological assessment.

Contraindications. Narrow or closed angles with risk of angle closure; hypersensitivity; phenylephrine 10 percent in infants, hypertension, coronary disease and aneurysm; atropine in Down syndrome (increased sensitivity) with care.

Effects. Photophobia, blurred near vision, stinging; acute angle closure attack in predisposed eyes (severe pain, red eye, nausea); systemic atropine effects (dry mouth, flushing, fever, tachycardia, urine retention, delirium, particularly in children and older adults); cyclopentolate may cause ataxia, disorientation and psychosis; phenylephrine may cause hypertension, tachycardia, arrhythmia and myocardial infarction; prolonged use of atropine causes allergic blepharoconjunctivitis.

Interactions. Monoamine oxidase inhibitors and tricyclic antidepressants with phenylephrine (hypertension); other anticholinergics (additive); miotics (opposed effect).

Monitoring. Warn patients not to drive for 4 to 6 hours (longer with cyclopentolate and atropine), to wear sunglasses, and to return urgently with severe eye pain or vomiting; observe children after cycloplegia. Use nasolacrimal occlusion to limit systemic absorption in infants.

Special groups. Premature infants: use 0.5 percent or less and phenylephrine 2.5 percent at most; feeding intolerance and apnoea can follow. Pregnancy: acceptable with occlusion. Older adults: assess for narrow angles and urinary retention. Region: dark irises need repeat or higher doses; routine dilated examination of diabetic patients is a main use, and angle closure risk is managed by checking the angle first.